1. Field of the Invention
This invention generally relates to electrophotographic copiers, and more specifically to those which are capable of providing selective operability for execution of either the regular standard copying mode or marginal image outline formation mode.
2. Prior Art
Generally speaking, the peripheral outline of an image is in practice full of necessary information thereabout and represents sufficiently the characterizing features of the image, thus playing among others a most important role in the judgement of the nature thereof. Further, it is to be noted that when a peripheral image outline is taken out and modified into a corresponding binary graphic representation, the latter has rather favorable characteristics in comparison with the corresponding regular, thick and page image representation, such as highly simplified and easier facilities for image identification; determination; transmission and the like. Therefore, the take-out of peripheral image outlines for conversion thereof into corresponding binary graphic representations is highly effective in image pattern recognition; image correction; image emphasis; band width compression and the like proceedings.
As an example, a complex color image pattern is rather attractive to realize by execution of two successive copying operations to provide a black pattern encircled by a color outline, or to prepare a blank pattern for the later producing differently colored local image areas contained therein.
It should be noted that in the name of the present assignee company, it has already been proposed to provide an outline image-forming process, wherein, in case of the mono-component type toner developing method using conductive toner to develop static latent images, said process is characterized in that, between the material to be subjected to developing and the carrier for the conductive toner, a d.c.-bias voltage is impressed at a medium potential ranging between the maximum and minimum surface potential of said material and having an opposite polarity to the static latent image charge, for extracting the outline marginal configuration only from the latent image on said material to be developed (refer to Japanese Open Patent Specification, unexamined, No. Sho-51-134635).
It should be further noted, however, that there is a considerable drawback in the above-mentioned prior art improvement. In this proposed process, the developed marginal outline image is only a negative one, because of the fact that the marginal outline of the static latent image is also negative and the conductive toner will be deposited onto a substantial part of the latent image other than the marginal outline thereof, and indeed, by virtue of higher potential difference charged at the substantive part of the latent image on the photosensitive drum. In practice, however, the wanted marginal outline should preferably be in black and thus positive. Therefore, the thus formed negative outline image must generally be subjected to a further reproducing step relying upon the reverse development principle, which represents naturally a grave and troublesome drawback inherent in improving the above known process.